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Introduct on

Man r o v e s a r e r e arded as major ecosystems due tot h e r ntermed a t e p o s t on between the mar n e a n d t e r -

r e s t r a l e n v r o n m e n t s , t h e r h h l e v e l o f p r mary pro-

duct v ty (Amaras n h e & B al a su b r a m a n am, 1992)

a n d t h e r n u t r t o n a l r o l e f o r c o a s t a l s h e l l f sh and f s h

populat ons (Lae d s a a r d & J o h n s o n , 1 9 9 5 ; K m a n et

a l . , 1 9 96 ; P r mavera, 1997). Worldw d e , t h e y c o v e r a n

a r e a o f n e a r l y 1 0 m l l o n h e c t ar e s , t h u s a c c o u n t n f o r

approx m a t e l y I % o f t h e s u r f a c e c o v e r e d b y t r o p c a l

f o r e s t s . T h e m a n r o v e s h a v e r e l a t v el y st r c t e c o lo -

c a l r e q u r e m e n t s ( e . h h a r t e m p e r a t u r e , p r e s e n c e

o f s h e l t e r e d a n d r e l a t vely shallow areas, w d e t d a l

r a n e , s h o r t p e r ods of m m e r s on and preferent al

p r e s e n c e o f m u d d y s u b s t r at e s ; Chapman, 1974) . A l s o ,

they are commonly unstable and often n r e r e ss on

a s a d r e c t re s u l t o f a n t hr o p o en c d s t u rb a n c e s , s u c h

a s t h o s e enerated by a q u a c u l t u r e a c t v t es, wh chd r e c t ly o r n d r e c t l y d e s t r o y t h e c a n o p y ( D e r b e r &K att s m u l , 1 9 9 6 ; B n h e t a l, 1 9 9 7 ) .

In Mada a s c a r , a q u a c u l t u r e n m a n rove areas

s t a r t e d r a t h e r t a r d l y , w t h t h e s e t t n up of a med um -

s c a l e s h r mp farm a lon t h e n o r t h - w e s t e r n c o a s t n t h e

be nn n of the 1990s (Mahajam ba bay) . I n o r d e r t o

m e a s u r e t h e e n v ronmental m p a c t o f t h s f a r m , a n a s -

sessment of the env ronmental cond t o n s b e f o r e t h e

o n s e t o f t h s a q u a c u l t u r e a c t v t y ( p r o d u c t on be n -

n n n O c t o b e r 1 9 9 3 ) w a s c o n s d e r e d e s s e n t a l , a s w a s

t h e p e r od c mon t o r n o f t h e e n v ronment dur n t h e

d u r a t o n o f t h s a c t v t y . F o r t h s t y p e o f e n v ronmental

a s s e s s m e n t , t h e u s e o f r e m o t e s e n s n p r o d u c e s f a s t ,

rel a b l e a n d p r e c s e r e s u l t s . T h e f r s t ob j ec t v e o f t h e

p r e se n t s t u d y w a s , th e re f o r e , t o d e t e rm n e t h e e n v r o n -

mental cond t o n s o f t h e p e r od preced n t h e s e t t n

u p o f t h e a q u a c u l t u r e a c t v t y u s n SPOT-XS da ta .

Hydrobiologia 413 : 127- 133, 1999 .

R . S. Dodd (ed.), Diversity a nd Function in Mangrove Ecosy stems .

1999 Kluwer Academ ic Publishers . P r i n t e d i n t h e N e t h e r l a n d s .

Mangrove ma pping in North-Wes tern Madagas car using

SPOT-XS and SIR-C radar data

Vanina Pasqualini 1 , 2 , Jacques l l t i s 3 , Nadine Dess ay 1 , Mar c L o i n t i e r 1 , O l i v i e r Guelorget4 &Laurent P o l i d o r i 1

1 I R D , M ais on d e l a T l d t e c t i o n , r u e J . F B r e t o n , 3 4 0 9 3 M o n t p e l l i e r , F r a n c e

2 U n i v e r s i t d e C o r s e , Equipe >, BY 52, 20250 C o r s e , France

T e l : [ + 3 3 ] 4 9 5 4 5 0 0 5 5 ; F a x : [ + 33 ] 4 9 5 4 6 2 4 4 1 ; E-Mail: pasq uali@univ-corse . fr3 IRD, Laboratoire d'Etudes R u r a l e s , BY 5045, 34032 Montpellier C e d e x , France

4 Lab oratoire d'Hydrobiologie Marine, UMR 5556 CNRS, Univers it M o n t p e l l i e r I I , C C 0 9 3, Pl. E . B a t a i l l o n ,34095 Montpe llier C e d e x , France

Ke y w ords : mangrove, Madagascar, m apping, remote sens ing, SPOT XS, SIR-C rada r

Abstract

Within the framework of an environmental impact study of a q u a c u l t u r e a c t i v i t i e s i n t h e c o a s t a l m a r s h o f M a -

hajamba (Nort

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12 8

Althou h t h e d a t a a c q u s t o n s y s t e m s o f m u l t

s u t a b l e t o o l s f o r m a p p n m a n r o v e s a n d a s s e s s n

t h e r o v e r a l l c o n d t on (Dutr e u x e t a l . , 1990 ; G r e e n

e t a l ., 1996, 1998 ; Lon & S ewes, 1996 ; Ramsey

& J e n s e n , 1 9 9 6 ) , t h e r u s e n t r o p c a l c o a s t a l a r e a s s

o f t e n l m t e d d u e t o a n a l m o s t p e r m a n e n t c l o u d c o v e r .T h u s , r e c e n t r e s e a r c h e f f o r t s h a v e t u r n e d t o t h e u s e

o f r e m o t e s e n s n r a d a r ( H e ry e t a l . , 19 9 3 ; Mou n e t

a l . , n p r e s s ; Pr o s y e t al . , n p r e s s ) , a s t h s ma e a c -qu s t on system s n o t h n d e r e d b y c l o u d c o v e r . B a s e d

on SIR-C radar data of the ma rsh of Mahajam ba, the

r a d ometr c behav our of the man r o v e s , a s e v a l u a t e d

from the r d e n s t es, was exam ned and compared to

o t h e r t r o p c a l p la n t f o rm a t ons . R e m o t e s e n s n o f t h eman r o v e s , u s n r a d ar d a t a , s e n e r a l l y c a r r e d o u tu s n a p u r e l y r a d ometr c approach. Themat c a n a -

l y s s u s n rad o m e tr c a n d t e x t u r a l c l a ss f c a t ons was

a l s o c o n s d e r e d h e r e n o r d e r t o d e f ne man rove map-

p n methods . T h e l o n term o a l o f t h s s t u d y s t o

e n e r a t e a f a s t a n d r e l a b l e t o o l t o b e u s e d n the mon t -

o r n of man rove hab t a t s s u b j e c t e d t o a n t h r o p o en c

a c t v t es .

Mater als and methods

The study s t e s l o c a t e d n North-Western Mada as-

c a r , b e t w e e n l a t tudes 15 10' S and 15 45' S, and

l o n tudes 46 45' E and 47 20' E (se e F u r e 1 ) .

Ma h a j a m b a b a y s t h e p o n t o f c o n v e r ence of two

lar e c o a s t a l r v e r s : the Sof a ( n t h e e a s t ) a n d t h e

Mah a j a m b a ( n t h e s o u t h ; s e e F u r e 2 ) . Mean annua l

prec p tat on s n t h e o r d e r o f 1 5 0 0 m m , o c c u r r n f o r

the most part between November and Apr l ( C h a p e r o n

et al . , 1 9 9 3 ) . L o c a l t dal ampl tude s 4 - 4 .50 m dur n

t h e s p r n t d e s .

SPOT XS ma e r y ( 2 0 m p xel), ta en on 21 Apr l1993 (10 h15 s tandard t me, t de of +2 . 0 2 m ) , and

composed of 3 rou h channels (XS3, XS2, XS 1), was

a n a l y z e d u s n GEO ma e software . A l o w l e v e l o fc l o u d c o v e r w a s p r e s e n t o n t h s d a t e , a l t h o u h some

clouds can be obs erved to the south-east and sm o e

s p e r c e p t b le n t h e c e n t r e o f t h e ma e . A p r e t r e a t -ment was f rst performed, the purpose of wh ch was

t o c a r r y o u t r a d om etr c and eometr c a l c o r r e c t ons

t o t h e r o u h ma e . T h e eometr c a l c o r r e c t on con-s s t s o f t r a n s f e r r n o n t o t h e ma e , a n d s u b s e q u e n t l yonto the reference document (topo raph c a l m a p s t o

the 1/100 000 : N-39, 1953; 0-39, 1956 ; N - 3 8 , 1 9 5 3 ;

0-38, 1953), several reference po n t s ; t h e r e l a t ve

coord nates of wh ch are recorded n each reference

m a r . These coord nate l s t s enerate a polynom al

model, ma n t p o s s b l e t o p a s s f r o m t h e r e f e r e n c e

m a r o n t h e ma e t o t h a t o f t h e m a p . T h e p r o j e c t onsystem used s of the Mada ascar Laborde type w th

n t e r n a t onal 1967 e l l psode . D s t n u s h n b e tw ee nt h e m m e r s e d a n d t e r r e s t r al areas was made poss b l e

u s n a p l a n t n d e x s o a s t o a v o d m s t a n s l h t l y

mmersed areas and man r o v e s . T h e t e r r e s t r a l z o n e s

w e r e m a p p e d u s n superv s e d c l a s s f c a t on (max-

mum probab l ty) w th XS3/XS2/Plant Index chan-

n e l s , b y s u c c e s s ve mas s o n t h e m a n roves, swam ps

a n d t e r r e s t r al ecosystems . Manual correct ons were

c a r r e d o u t b a s e d o n f e l d d at a ob t a ned between 1993

and 1995 .

SAR radar ma ery (Synthet c Aperture Radar)f r o m t h e s a t e l l te ERS-1, type SIR-C (Spaceb orne

I m a n R a d a r - C ) , t a en on 7 October 1994 (11 h15

s t a n d a r d t m e , t de of +0 . 3 4 m ) , p r e s e n t s a p o r t on of

t he s t ud y s t e , an d n p ar t c u l a r t h e m o u t h o f t h e S o f a

and Mahaj amba r v e r s . T h s m u l t frequency data was

obta ned as C-band (5 .35 GHz or 6 cm) and L-band

( 1 .25 GHz or 23 cm) w th VV and VH polar za t on

( R e s o l u t on : 1 5 m ; W d t h o f s w e e p : 35-100 m; V ewa n l e : 15-60 ; L e n t h o f a e r a l : 12 . 1 m) . E a c h m -

a e p x e l r e p r e s e n t s t h e a m p l t u d e o f t h e r e t r o d f f u s e d

s n a l . T h s numer cal data (DN) was trans formed so

a s t o o b ta n t h e re t r od f f u s on coeff c ents (dB) f o r

e a c h p xel (dB=-40 .2+0 .2DN ; Pers . comm : NASA) .A f l t e r n was then appl e d t o t h e ma e s n o r d e r t or e d u c e t h e s h m m e r p h e n o m e non ( s p e c le) . Themat c

a n a l y s s u s n r a d o m e t r c c l a s s f c a t on was carr ed

o u t b y d r e c t e d c l a s s f c a t on (max mum probab l t y ) ,

o n t h e b a s s of both the L-band assoc ated w th two

p o l a r za t ons and the C-band assoc ated w t h c r o s s e d

p o l a r z a t on, namely L-VH, L-VV and C-VH . Them-

at c a n a l y s s u s n t e x t u r a l c l a s s f c a t on was carr ed

o u t u s n the L-band assoc ated w t h c r o s s p o l a r z-

a t on and the PAPRI softwa re (Borne, 1990 ; CPIS,

1996) . The cho ce of the L-VH channel s e x p l a ned

b y t h e f a c t t h a t l o w f r e q u e n c es cross natural med -

ums w t h m o r e e a s e , a n d c r o s s p o l a r z a t o n a c c e n t u -

ates d s c r m n a t o n b e t w e e n t h e p l a n t f o r m a t ons . Th sm e t h o d m a e s t p o s s b l e t o t h r e s h o l d t h e ma e na superv sed manner, based on the textural hetero-

ene t y o f t h e s t r u c t u r e s . Ima e a n a l y s s was carred

o u t u s n a p xel w ndow . F o r a ven w ndow, the

c l o s e s t l a n d s c a p e a n d t h e d s t a n c e ( d stance between

t h e t h e o r e t cal compos t on of the landscape and the

real compos t o n o f t h e w ndow) were ca lculated . I m -

a e a n a l y s s w a s c a r r e d o u t o n a s e t r a n e o f w ndow

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1 2 9

F ure 2 . Mapp n o f t h e t r o p c a l p l a n t f o r m a t ons by ma e process n of SPOT- XS : (A) of Mahajamba b ay, (B) of the area surround n t h es h r mp farm and (C) at the mouth of the Mahaja mba and Sof a r v e r s .

F u r e 1 . G e o raph c a l l o ca t o n o f t h e s t ud y s te

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1 3 0

s z e s , n o r d e r t o p r o v d e t h e b e s t p o s s b l e c la s s f c a -

t on of each structure to be mapped . T h r e e t e s t s a r e

real s e d w t h 3 x 3 a n d 5 x 5 w ndows, w t h 2 9 x 2 9 a n d

3 1 x 3 1 w ndows a nd from 3 x 3 to 31x31 . A eomet-

r c a l c o rr e c t o n w a s t h e n c a r r ed out based on ma ereference po n t s u s n the SPOT data and th s was

done n o r d e r t o r e n d e r t h e m s t r c t l y superposa ble .

Results a nd d s c u s s on

The analys s of SPOT-XS ma ery made t p o s s b let o m a p Ma h aj a mb a b a y n A pr l 1993 and revealed

t h e p r e s e n c e o f s e v e r al d f f er e n t e n v ronments : m a n -

rove ecosystems, terrestr a l e c o s y s t e m s (savannahs

and forests) and swampy areas (see F ure 2A) . T h e

surface area covered by the coastal marsh s 5 6 4 0 0

ha, of wh ch 43 700 ha s m a d e u p o f m a n r o v e s .

T h e s e m a n roves are w ell developed n t h e s o u t h e r n

a n d e a s t e r n r e o n s o f t h e b a y , a t t h e m o u t h o f t h e

So f a r ver and n t h e w e s t e r n p a r t s . Some small man-

rove s l a n d s a r e p r e s e n t t o t h e n o r t h . F o u r t y p e s o fman roves have been dent f e d : t w o t y p e s f r o m m o r -pholo c a l p a r am et e r s ( f r o n t al , n t e rf lu v a l ) a n d t w o

other types from dynam c p a r a m e t e r s ( m a t u r e , r e c e s s -

v e ) . T h s z o n a t on s c h a r a c t e r s t c of the man rove

ecosystem's dynam c s n Mada a s c a r ( L eb r e, 19 9 0 ) .

T h e l e v e l s o f f l u v a l a n d s e d mentary nput determ nes

t he s p at a l e vo l u t on of the man r o v e s . A t t h e m o u t h

o f t h e S o f a r ver, n t h e e a s t e r n p a r t o f t h e b a y , f l u v a l

nput s h h a n d t h e ma n rove m a p s h ow s n t e n s v e

hydrosed m e n t a r y d y n a m c s n t h s r e on (see F -

ure 2B) . L a r e q u a n t t e s o f l a t e r te mud accumulate

n the shallow waters and almonds of sed ments are

f o r m e d a s o n e a p p r o a c h e s t h e s e a . P oneer man r o v e s

(e . . Sonnerat a e l b a , A v ce nn a m a r n a ) c a n t h e n d e -

v e l o p , a l l o w n a f a s t p r o p a a t on . In the mouth of

the Sof a , t h e m a t u r e m a n r o v e s (Rh zophora spp . ) a r e

w e l l d e v e l o p e d ( s e e F u r e 2 B ) , r e v e a l n t h e n t e n s ve

dynam c s p r e s e n t a t t h s s te . A p r e v o u s s t u d y o f t h e

s p a t otemporal evolut on of man r o v e s n the mouth

of the Sofa would tend to confrm th s r e s u l t ( L e b -

r e , 19 9 0 ) . C on v e r s e l y , n t h e c e n t ra l p ar t o f b ay , at

the mouth of the Mahajamb a, mature man r o v e s a r e

l e s s d e v e l o p e d ( s e e F ure 2B), wh ch would su e s t

more reduced dynam c s . Most of the wate rs of the

Ma h a j a m b a b a s n a r e a r t f c a l l y c o n t a ned upstream

o f t h e c o a s t a l p l a n (Chaperon et al . , 1993) . Bey-

ond the mature man roves and n t h e r e o n o f t h e

n t e r f l u v al man r o v e s , m a n y s a l t m a r s h e s ( b r n e e c o -

systems w t h o u t v e etat o n ) d e v e l o p ( s e e F ure 2C) .

In Apr l 1 9 9 3 , t h e f r s t o f t h e s hr m p f ar m n ponds

was set up n one of these marshes (see F ure 2C)Th s a q u a c u l t u r e a c t v ty currently occup e s s e v e r a lh u n d r e d h e c t a r e s , n s t a l l e d p r m a r l y o n s a l t m a r s h e s

and some man r o v e a r e a s .

The SPOT-XS data ma de t p o s s b l e t o c h a r a c -

ter ze the Mahajamba bay env ronment pr o r t o t h e

development of the a q u a c u l t u r e ponds . T h e s e d a t a w l l

be used to mon t o r t h e e v o l u t o n o f t h s r e on over

t me. However, the use of SPOT data presents sev-e r a l d sadvanta es . F r s t l y , t h e u s e o f t h s d at a s n ot

always poss ble, due to an almost permane nt cloud

cover. The s ze of the p xel (20 m n mult s p e c t r a l

m o d e ) a l s o r e p r e s e n t s a l m t n f a c t o r n t h e p r e c s emon tor n of man r o v e e v o l u t on . L a s t l y , t h e m a p -p n o f t r o p c a l a re a s u s n SPOT data can lea d to a

number of confus ons and requ r e s t h a t a c l a s s f c a -

t o n u s n m a s s be performed wh c h t e n d s t o c o n f e r

a lar e a m o u n t o f s u b j e c t v t y t o t h e m a n p u l a t o r . Th e

rad o me tr c be h av our of th e m a n r o v e s , a s d e t e r m -

n e d u s n S I R - C d a t a , h a s , t h e r e f o r e , b e e n d e t e r m ned

based on the dens ty of these ecosystems, wh ch can

be compared to the other plant format ons present .

The var at on n m a n r o v e d e n s t y s e as l y d e te c t ab l e

u s n S I R - C r a d a r d a t a ( P r o s y e t a l . , n p r e ss ) . T h s

a n a l y s s (s ee F u re 3 ) , c ar r e d o u t o n t h e f o u r c h a n -

nels ava l a b l e , r e v e a l s a s m l a r e n e r a l t e n d e n c y nt h e c h a n n e l r e s p o n s e f o r t h e f o u r c h a n n e l s c o n s dered .

The use of cross polar z a t on, however, accentuates

th s d s c r m n a t on between the plant format o n s a tlower frequenc es . For the L-band, the dense man-

r o v e s a r e e a s l y d e n t f a b l e . A s f o r t h e d f f u s e a n d

r e r e s s ve m a n r o v e s , t h e y c a n b e d s t n u shed only

on the L-VH channel . The C-band would appear to

be of n t e r e s t n d f f e r e n t a t n between forests and

s wa m py m e a do ws . It seems that these answers a re

nfluenced by the water or hum d t y p r e s e n t b e n e a t h

t h e p l a n t f o r m a t ons and w ll d f f e r a c c o r d n t o t h e

wave len th em t t e d . S a v a n n a h s a r e e a s l y dent f a bl e

r e a r d l e s s o f t h e f r e q u e n c y o f t h e s n a l a n d e v e n u n -

der HH polar z a t o n ( P r o s y e t al . , n p r e s s ) . M o r e o v e r ,

t h e va l u e of t h e re t r od f f us on coeff c e n t n c r e a s e s a s

a f u n c t o n o f t h e d e n s t y o f th e p l a n t f o rm a t o n s ( s e e

F u r e 3) a n d th u s as a f un c t o n o f b omass (Pro s y e t

al . , n p r e s s ) .

On the bas s o f t h e s e o b s e r v a t ons, themat c a n a -

l y s s u s n r ad o m etr c c l a s s f c a t on was carr ed out

u s n the L-VH, L-VV and C-VH channels (see F -

u r e 4 ) . T h e s e r e su l t s r e v e al a c e r t a n amount of confu-

s on n t he n t e rp r e ta t on . I n d e e d , r e c e s s ve man r o v e s

cannot be d s t n u s h e d f r o m t h e f o r e s t s , n o r t h e d f -

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fuse man r o v e s f r o m s a v a n n a h s , t h e s a l t m a r s h e s f r o m

s w a m p y m e a d o w s a n d w a t e r f r o m s w a m p y m e a d o w s .

T h e m a t c a n a l y s s u s n r a d o m e t r c c l a s s f c a t on of

t h e S I R - C r a d a r d a t a , t h e r e f o r e , d o e s n o t a p p e a r t o

b e a rel a b l e m e t h o d f o r p r e c sely mapp n man r o v e sa n d o t h er t r op c a l p la n t f o rm a t ons . Themat c a n a l y s s

u s n t e x t u r a l c l a s s f c a t o n w a s t h u s c a r r e d o u t u s n

only the L-VH channel (see F u r e 5 ) . T h e te s t c a r -

r e d o u t , w t h 3 x 3 a n d 5 x 5 w ndows (see F u r e 5 A ) ,

prov de s n t e r e s t n r e s u l t s n t e r m s o f t h e dent fc a -t o n o f d e n s e m a n roves, water and swampy meadows ,w t h v al u e s n t h e c o n f u s on matr x h her than 84% .U s n 29x29 and 31x31 w ndows (see F ure 5B),

m o s t o f t h e s t r u c t u r e s c a n b e dent f ed, w t h t h e e x -

c e p t o n o f f o r e s ts a n d s al t ma r s h e s . A s t h e s a l t m a r s h e s

1 3 1

F u r e 3 . Radar responses concern n the trop cal plant format ons n the L- band and C-band assoc a t e d w t h V H a n d V V p o la r -

z a t ons (Retrod f f u s on coeff c ent depends on the ampl tude of retrod ffused s nal and s expressed by the relat on : retrod f f u s on

coeff c ent=(K* R4*Pr)/Sr, w th K=wed n coeff c ent, R=c b l e d stance, Pr=rece ved power, Sr=area of observed surface) .

a r e t o t a l l y c o n f u s e d w t h w a t e r , t s a s s u m e d t h e y h a v e

the same texture . T h e f o r e s t s , n places, confusedw t h t h e r e c e s s ve man r o v e s ; t h e y t h u s h a v e t h e s a m et e x t u r e a n d , c o n s e q u e n t l y , t he s a m e d e n s t y . M o r e o v e r ,

t h e d e l m tat o n s o f t h e d f f e r e n t p l a n t f o r m at o n s a r en o t v e r y d s t n c t (s ee F u r e 5 B ) . C o n s e q u e n t l y, a th r d

t e s t s c o n s d e r e d u s n a l a r er w n d o w f r o m 3 x 3 t o

3 1 x 3 1 ( s e e F ure 5C) . Th s t r e a t m e n t a l l o w s u s t o

m o r e c l e a r l y d s t n u s h t h e d e l m ta t o n s o f t h e p l a n tformat o n s , b u t d o e s n o t r e d u c e t h e o v e r l a p p n o f s alt

m a r s h e s a n d f o r e s t s . Themat c a n a l y s s u s n t e x t u r a l

c l a s s f c a t o n o f S I R - C r a d a r ma e s , th e re f or e , e n e r -ates prom s n r e s u l t s n t e r m s o f t h e p r e c se mapp n

of man r o v e s an d o t h e r t r op c a l p la n t f or m at o n s , w t h

t h e e x c e p t o n o f f or e s t s a n d s a l t m a r s he s .

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1 3 2

C on clu s o ns

The m a pp n o f Ma h a j a m ba b a y , u s n SPOT-XS

data, made t p o s s b l e t o c h a r a c t e r z e t h e e vo l u t on -

a r y d y n a m cs of the man roves present w th n t h e

bay pr o r t o t h e s e t t n up of a q u a c u l t u r e r e a r n

F ure 4 . Mapp n o f t h e t r o p cal plant format ons n the mouth of the Mahajamba and Sof a r v er s u s n rad ometr c c la ss f c a t on of SIR-Cradar ma e s

F u r e 5 Mapp n o f t h e t r o p cal plant format ons n the mouth of the Mahajamba and Sof a r v e r s u s n t e x t u r a l c l a s s f c a t on of SIR-C radarma e s : ( A ) w th 3x3 and 5x5 w ndows, (B) w th 2 9x29 and 31x31 w ndows, (C) from 3x3 to 31x31 .

ponds . The mouth of the Sof a r v e r s c h a ra c t e r zed

by n t e n s v e d y n a m c cond t ons n t s f r o n t a l a r e a .

C o n v e r s e l y , t h e c e n t r a l p a r t o f t h e b a y e x h b t s a mo r e

r e d u c e d e v o l u t on . A n a l y s s o f t h e r a d ometr c beha-

v our of the ma n roves and other plant format ons

n Mahajamba bay, as determ ned from SIR-C radar

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d a t a, r e v e a l s t h a t t h e s e f o r m a t ons are more eas l y

dent f ed when the frequency used s low and when

p o l a r z a t on s c r o s s e d . Themat c ma pp n u s n r a -

d ometr c c l a s s f c a t on d d not e n e r a t e s a t s f a c t o r y

r e s u l t s . C o n v e r s e l y , t e x t u r a l c l a s s f c a t on us n t h e

L-VH channel prov d e d n t e re st n r e s u l t s, n s o f a r a s

t h e p r e c se mapp n of man r o v e s a n d o t h e r t r o p c a l

p l a n t f o r m a t ons s concerned . R e s u l t s f o r f o r e s t s a n d

s a l t m a r s h e s u s n th s t e c h n que, however, were not

sat s f a c t o r y . In order to mon t o r t h e e n v ronmental

e v o l u t on of man r o v e s n r e l a t o n t o t h e d e v e l o p m e n t

o f a q u a c u l t u r e a c t v t e s n Mahajamba bay, the tex-

t u r a l a n a l y s s of SIR-C rada r ma es appears to bet h e t e c hn q u e m o s t a d a p t e d t o o u r o b j e c t v e s , w t h t h e

added a dvanta e o f p os s e ss n a sh or t t re at me n t t me.Ac n ow le d e me nt sWe ma e a p o n t o f t h a n n C h r s t an Reynaud, re-s e a r c h e r s a t t h e L a b o r a t o r y o f M a r ne and Cont n e n t a l

H ydro b o l o y n Montpell e r (F r an c e ), f or t he n t er e st

they showed n t h s wor . O u r t h a n s a l s o o t o F re -

d e r c Borne, researcher at the CIRAD Laboratory,

Centre f or Remote Sens n (Montpell e r ) a n d A ndr

Beaudo n, researcher at the CEMAGREF Laboratory,

Centre for Remote Sens n , f o r the r v a l u a b l e a d v ce

a n d h e l p n t h e c o m p l e t o n of t h s s tu d y .

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product v ty of two man rove forest stands on the northwestern

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B nh, C . T . , M . J . P h l l p s & H . D e m a ne, 1997 . I n t e r a t e d s h r mp-man rove farm n systems n the Me on delta of V e tna m .Aquacult . R e s . 2 8 : 599-610

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t r o p c a l c o as t a l r e s o u r c es . C o a s t. M mt 24 : 1-40 .Hery, P. , D . Ducrot-Gambart, A. Lopes, E . Mou n, G . Marty, F .

Fromard, F . Blasco, J . P. Rudant, M . Lo n t er & M . T . P r o s t ,1993 . C arto ra ph e d e l a v t a t on trop c a l e et su v d e l a d y -nam que des man roves de G uy a ne p ar SPOT e t E RS -l . InCpadus eds, De l ' o p t q ue a u r a d a r , l e s applcat ons de SPOT etERS, Ma 1993, Par s , F r a n c e : 329-341 .K rnan , E . N . , G. K . Mwatha, E . O . Wa wab , J . M . N t h a & B .K . O oth, 1996 . F shes of a shallow trop cal man r o v e e s t u a r y ,G a z , Kenya . Mar. Freshwat. Res . 4 7 : 857-868 .

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man roves w th Landsat TM satell te data and Geo raph c I n -format on System . E s t u a r . c o a st . s he lf . S c . 4 3 : 373-381 .

Mou n , E . , C . P r o sy, G . M a r t y , F . Fromard, H . P u , J . L . B e t o u l l e& J . P . Rudant, 1999 . Mult frequency and mult polar z a t on radar

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Pr mavera, J . H . , 1 9 9 7 . F sh predat on on man rove-assoc ated pen-

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t o n radar des man roves de Guyane Frana s e . Photo- n t e r p r e t . ,I m a e s spat a l e s e t a r ennes, num . S p c . n p r e s s

Ra m s e y , E . W . & J . R . Jense n, 1996 . Remote sens n of man-

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